Mast cells play important roles in inflammation and innate immune reactions. Activated mast cells release many inflammatory mediators. Some of them are preformed, stored in their secretory granules, and released via degranulation, a form of regulated exocytosis. The importance of degranulation, different to other mast cell responses, in the development and evolution of inflammation is not known. It is accepted that mast cell degranulation should be important in the early allergic response because histamine and serotonin are secreted through this mechanism. However, we have recently shown that mast cell tryptases released during degranulation play important roles in inflammatory arthritis and defense against bacterial infections. The effects of degranulation on the chronic stages of allergic and non-allergic inflammation are not known. Exocytosis is a highly regulated process, and Syntaxins and Munc18 proteins are essential components of the exocytic machinery. Although the precise role of Syntaxins in exocytosis is well known, the identity of the exocytic step controlled by Munc18 proteins remains controversial. Syntaxins-3 and -4, and Munc18-2 and -3 are the main isoforms expressed in mast cells. Munc18-3 regulates Syntaxin-4 and Munc18-2 regulates Syntaxin-3. We have created genetically-modified mice with specific deletions of each of these four genes in their mast cells. Preliminary studies in these mutant mice point to a severe and selective defect in mast cell regulated exocytosis. We plan to study single mast cells from these unique mice at high resolution to uncover the specific exocytic step mediated by each of these Syntaxins and regulated by each of these Munc18 proteins. Additionally, we will be able to describe how two different Syntaxin/Munc18 pairs control the same vesicular trafficking event. Then, moving from single cell to whole animal studies, we will try to end the controversy about the extent and versatility of the effects of mast cell degranulation by studying our mast cell degranulation- deficient mice in models of chronic allergic asthma, inflammatory arthritis, and bacterial pneumonia. Our genetically modified mice may reveal if mast cell degranulation is an attractive and safe therapeutic target for some inflammatory diseases.